New insights into the application of the valence rules in Zintl phases—Crystal and electronic structures of Ba{sub 7}Ga{sub 4}P{sub 9}, Ba{sub 7}Ga{sub 4}As{sub 9}, Ba{sub 7}Al{sub 4}Sb{sub 9}, Ba{sub 6}CaAl{sub 4}Sb{sub 9}, and Ba{sub 6}CaGa{sub 4}Sb{sub 9}

Journal Article · Fri Apr 15 04:00:00 EDT 2016 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/J.JSSC.2015.07.015· OSTI ID:22577816

He, Hua; Stoyko, Stanislav; Bobev, Svilen

Crystals of three new ternary pnictides—Ba{sub 7}Al{sub 4}Sb{sub 9}, Ba{sub 7}Ga{sub 4}P{sub 9}, and Ba{sub 7}Ga{sub 4}As{sub 9} have been prepared by reactions of the respective elements in molten Al or Pb fluxes. Single-crystal X-ray diffraction studies reveal that the three phases are isotypic, crystallizing in the orthorhombic Ba{sub 7}Ga{sub 4}Sb{sub 9}-type structure (space group Pmmn, Pearson symbol oP40, Z=2), for which only the prototype is known. The structure is based on TrPn{sub 4} tetrahedra (Tr=Al, Ga; Pn=P, As, Sb), connected in an intricate scheme into 1D-ribbons. Long interchain Pn–Pn bonds (d{sub P–P}>3.0 Å; d{sub As–As}>3.1 Å; d{sub Sb–Sb}>3.3 Å) account for the realization of 2D-layers, separated by Ba{sup 2+} cations. Applying the classic valance rules to rationalize the bonding apparently fails, and Ba{sub 7}Ga{sub 4}Sb{sub 9} has long been known as a metallic Zintl phase. Earlier theoretical calculations, both empirical and ab-initio, suggest that the possible metallic properties originate from filled anti-bonding Pn–Pn states, and the special roles of the “cations” in this crystal structure. To experimentally probe this hypothesis, we sought to synthesize the ordered quaternary phases Ba{sub 6}CaTr{sub 4}Sb{sub 9} (Tr=Al, Ga). Single-crystal X-ray diffraction work confirms Ba{sub 6.145(3)}Ca{sub 0.855}Al{sub 4}Sb{sub 9} and Ba{sub 6.235(3)}Ca{sub 0.765}Ga{sub 4}Sb{sub 9}, with Ca atoms preferably substituting Ba on one of the three available sites. The nuances of the five crystal structures are discussed, and the chemical bonding in Ba{sub 7}Ga{sub 4}As{sub 9} is interrogated by tight-binding linear muffin-tin orbital calculations. - Graphical abstract: The new Zintl phases—Ba{sub 7}Al{sub 4}Sb{sub 9}, Ba{sub 7}Ga{sub 4}P{sub 9}, and Ba{sub 7}Ga{sub 4}As{sub 9}, and their quaternary variants Ba{sub 6}CaTr{sub 4}Sb{sub 9} (Tr=Al, Ga)—crystallize in the Ba{sub 7}Ga{sub 4}Sb{sub 9} structure type. The structures are based on TrPn{sub 4} tetrahedra (a perspective of the crystal structure is shown, as viewed along the c axis). - Highlights: • Ba{sub 7}Al{sub 4}Sb{sub 9}, Ba{sub 7}Ga{sub 4}P{sub 9}, and Ba{sub 7}Ga{sub 4}As{sub 9} are new compounds in the respective phase diagrams. • The quaternary phases Ba{sub 6}CaTr{sub 4}Sb{sub 9} (Tr=Al, Ga) show nearly ordered Ba/Ca distribution. • Very weak Pn–Pn bonds and pairing distortion are observed for Ba{sub 7}Ga{sub 4}Pn{sub 9} (Pn=P, As).

• OSTI ID: 22577816
• Journal Information: Journal of Solid State Chemistry, Journal Name: Journal of Solid State Chemistry Vol. 236; ISSN 0022-4596; ISSN JSSCBI
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
ANTIMONY
ARSENIC
BARIUM IONS
CATIONS
CHEMICAL BONDS
ELECTRONIC STRUCTURE
LAYERS
MONOCRYSTALS
MUFFIN-TIN POTENTIAL
ORTHORHOMBIC LATTICES
PHASE DIAGRAMS
PNICTIDES
SPACE GROUPS
X RADIATION
X-RAY DIFFRACTION